Over the years people have developed various sophisticated confocal interferometric techniques. Examples of the variety of the available technologies are the following.
There is interferometric, confocal far-field and near-field microscopy using heterodyne techniques and a detector having a single detector element or having a relatively small number of detector elements.
There is also interferometric confocal far-field and near-field microscopy using a step and stare method with a single-homodyne detection method for acquiring conjugated quadratures of fields of reflected and/or scattered beams when a detector is used that includes a large number of detector elements. The respective conjugated quadrature of a field is |α|sin φ when the quadrature x(φ) of a field is expressed as |α|cos φ. The step and stare method and single-homodyne detection method have been used in order to obtain for each detector element a set of at least four electrical interference signal values with a substrate that is stationary with respect to the respective interferometric microscope during the stare portion of the step and stare method. The set of at least four electrical interference signal values are required to obtain for each detector element conjugated quadratures of fields of a measurement beam comprising a reflected and/or scattered far-field or near-field from a spot in or on a substrate that is conjugate to the each detector element.
There are heterodyne and single-homodyne detection methods to obtain phase information in linear and angular displacement interferometers.
And there is a double homodyne detection method based on use of four detectors wherein each detector generates an electrical interference signal value used to determine a corresponding component of a conjugated quadratures of a field such as described in Section IV of the article by G. M D'ariano and M G. A. Paris entitled “Lower Bounds On Phase Sensitivity In Ideal And Feasible Measurements,” Phys. Rev. A 49, 3022-3036 (1994).). The four detectors generate the four electrical interference signal values simultaneously and each electrical interference signal value contains information relevant to one conjugated quadratures component.
High speed, high resolution imaging with high signal-to-noise ratios is required, for example, in inspection of masks and wafers in microlithography. Two techniques that have been used for obtaining high resolution imaging with high signal-to-noise ratios are interferometric far-field and near-field confocal microscopy of the types described above. However, the acquisition of high signal-to-noise ratios with the high resolution imaging generally limits data rates in part by the necessity to acquire conjugated quadratures of fields of a reflected and/or scattered beam for each spot in and/on a substrate being imaged. The determination of conjugated quadratures requires the measurement of at least four electrical interference signal values for the each spots in and/or on the substrate being imaged. Acquisition of the at least four interference signal values for the each spots places tight restrictions on how large a rate of scan can be employed in generation of a one-dimensional, a two-dimensional or three-dimensional image of the substrate having artifacts down to of the order of 100 nm in size or smaller.